Common-Mode Current in EFHW and Off-Centre-Fed Antennas
Common-Mode Current in EFHW and Off-Centre-Fed Antennas
Does an off-centre-fed antenna put less RF on the coax than an end-fed half-wave? It can—but the antenna name cannot prove it. The useful comparison begins with the intended return conductor, the current on the coax exterior and the boundary where that current should end.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
The practical concern behind this comparison is real. Exterior-feedline current can alter the installed pattern, carry transmitted RF toward the station and nearby wiring, or carry local noise back toward a receiving antenna. But “EFHW,” “OCFD” and “multiband” are geometry labels—not measured common-mode-rejection figures.
My practical rule: draw the wanted differential path and the complete RF return before choosing a transformer or choke. Transformation and common-mode suppression are separate functions. Measure exterior current at several positions, then put the choke at the boundary the installed current path actually needs.
What Current on Coax Are We Discussing?
Coax supports more than one current pattern. In the wanted transmission-line mode, current on the centre conductor is accompanied by equal-and-opposite current on the inner surface of the shield. Their external fields are substantially confined by the cable geometry. A separate current can flow on the shield’s outer surface and through whatever conductors and capacitances complete that external loop.
Roy Lewallen, W7EL, calls that exterior component an imbalance current in Baluns: What They Do and How They Do It. It can radiate, receive, change feedpoint behaviour and make the feedline part of the installed antenna. Measuring SWR alone cannot tell how much of that exterior current exists.
The word common mode is often used for every current found on a cable shield. That can hide an important distinction: a declared section of coax exterior may be an intentional antenna branch, while current continuing beyond its designed boundary is unwanted. The instrument sees current in both cases; the design intent tells us which section belongs to the antenna.
An End-Fed Wire Still Needs a Return
An EFHW presents a conductor end to its matching network, but the source still needs a second current path. Depending on the installation, that return can include a deliberate counterpoise wire, a declared section of coax exterior, a mast, transformer and enclosure capacitance, ground coupling, bonding conductors, station wiring or several parallel branches.
If no return branch is drawn, it has not disappeared. The environment supplies one. Tom Rauch, W8JI, demonstrates this directly in his measured analysis of end-fed half-wave systems: matching-network capacitance and everything attached to the feed system can take part in current closure.
This is why placing a choke directly beside an end-fed transformer is not automatically a cure. If the coax exterior was the only practical return, the choke changes the antenna circuit rather than merely removing a nuisance. Provide or identify the intended return first; then use the choke to define where that branch ends.
An Off-Centre Feed Does Not Guarantee Balance
A conventional off-centre-fed dipole has two intentional wire arms connected at an asymmetric point. Compared with feeding near a current minimum at the end of a half-wave wire, its antenna-side impedance can be more moderate on some bands. That can simplify one matching problem. It does not establish common-mode current.
The coax sees a three-dimensional installation: unequal arm geometry, unequal capacitance to ground and structures, a support or mast, a transformer, the feedline route and the station termination. Even when the intended arm currents are driven as a differential pair at the feedpoint, those asymmetries can excite a separate exterior-feedline path. A suitable current balun or choke at the feedpoint is therefore a sensible first candidate when the two wire arms are meant to be the complete radiator—but its result still has to be measured.
A one-end off-centre-fed architecture is a different circuit. It can deliberately use the coax exterior between an impedance transformer and a choke as the shorter radiating branch. In that case the current before the choke is intentional; the question is how much continues beyond the boundary. Calling that arrangement an OCFD without drawing the coax branch erases the most important conductor.
| Installed architecture | Intended return | Common-mode question |
|---|---|---|
| EFHW-like end-fed wire | A separate wire, declared coax-exterior section, mast/environmental coupling or a documented combination | Which branch closes the source current, and where should exterior current stop? |
| Conventional coax-fed OCF dipole | The second wire arm is part of the intended differential radiator | How much exterior current is created by feedpoint and installation asymmetry? |
| One-end EF-OCF arrangement | A declared coax-exterior section before a separately defined choke boundary | Does the intended short branch carry the expected current, and how much remains after the choke? |
None of those rows contains a universal number of decibels. The result belongs to the actual frequency, geometry, load, transformer, choke, cable route and surroundings.
High Feedpoint Impedance Is Not a CMC Rating
Near the end of a resonant half-wave conductor, current can be relatively low and voltage relatively high. That helps explain why an EFHW normally needs a substantial impedance transformation and careful voltage design. It does not mean return current is zero, nor does it tell us what fraction flows on the coax exterior.
The matching network is not an ideal ratio label. Winding capacitance, leakage coupling, enclosure capacitance, lead layout, core behaviour and the impedance connected to each terminal all influence the external current loop. A lower nominal ratio is not automatically quieter, and a higher ratio is not automatically the cause of every feedline-current problem.
Likewise, an OCF feedpoint’s more moderate resistance on one band does not guarantee low exterior current. The useful quantities are the complex differential load, the common-mode source and termination impedances, and current measured along the installed exterior path.
Multiband Coverage Must Be Rechecked on Every Band
An 80–10 m label can create the impression that one transformer, one choke and one coax length behave the same way across HF. They do not. The wire supports different current modes, the antenna-side R + jX changes, the exterior branch has a different electrical length, and the choke’s complex impedance changes with frequency.
A low transmitter-side SWR on several bands establishes only the match at that reference plane. It does not prove low transformer loss, low feedline loss, a controlled return branch, low exterior current or a useful radiation pattern. A multiband EFHW does not have to use an uncontrolled coax radiator, but the return must be explicit and verified. A multiband OCFD likewise does not become balanced merely because several SWR minima appear.
Do not divide the market into a “good” off-centre family and a “bad” end-fed family. A deliberate, measured EFHW can keep current out of the station; a poorly installed OCFD can put substantial current on its coax. The topology tells you which questions to ask, not the answer.
Do Not Choose Chokes From a Fixed Recipe
A fixed 0.05-wavelength location and mandatory two- or three-choke recipe cannot stand in for an installed current analysis. A physical cable length is a different fraction of a wavelength on every band, and the propagation of the exterior mode is not automatically described by the manufacturer’s differential-mode velocity factor.
Start from the intended boundary:
- Complete two-arm radiator at the feedpoint: a characterised feedpoint choke is a reasonable first trial because the coax exterior is not intended to be a third radiator.
- Deliberate coax-exterior return branch: the choke belongs at the far end of that declared branch, after current and pattern have been checked on every operating band.
- Current reappears farther down the route: investigate mast, control-cable, building and station coupling before adding another choke.
- Current reaches equipment: an entry or equipment-end choke may control a second loop, but it cannot repair an undefined antenna return by itself.
An UNUN can provide the impedance transformation required by an intentionally unbalanced port. A separately characterised 1:1 choke can define the common-mode boundary. That is my practical default when the measured system calls for both jobs, not a universal ratio or an assertion that every antenna is unbalanced. A suitable current balun remains valid for a genuinely balanced installed load.
A “30 dB Choke” Is an Incomplete Specification
Decibels require a measurement definition. A fixture S21 value is not automatically the reduction in exterior current after installation. The installed suppression depends on the choke’s complex common-mode impedance and on the source and load impedances of the exterior-current loop.
ZCM(f) = RCM(f) + jXCM(f)
The resistive and reactive parts both influence current. Ferrite properties, turns, cable geometry and parasitic capacitance make them frequency-dependent. The same assembly can be predominantly resistive in one region, reactive in another and limited by a winding resonance elsewhere.
The Fair-Rite technical catalogue publishes complex, frequency-dependent material behaviour—not a universal “Type 31 from 1 to 30 MHz” guarantee for every winding. Core size alone does not establish saturation, heating, voltage withstand or differential cable stress. The completed choke has to be measured and qualified for its actual load, power, waveform, duty cycle and cooling.
Close Neighbours Make the Current Path More Important
Exterior-feedline current can increase the field near house wiring, data cables and connected equipment. That can make interference more likely, but an interference report does not identify the coupling path by itself. The field may come from the intended antenna, the coax exterior, another station cable, conducted RF or a susceptible device acting as an unintended receiver.
Keep the investigation constructive. First verify that the transmitter, amplifier and accessories are operating cleanly. Then map current on the coax, mast, control and station cables; change one coupling path; restore the baseline; and repeat. A choke that lowers one neighbour’s symptom may have changed exterior current, the intended antenna field or both.
Do not disconnect protective-earth conductors or improvise unsafe grounds during these tests. Earthing, equipotential bonding, lightning and surge protection, cable-entry shielding and RF common-mode control are coordinated but distinct jobs. IEC TR 61000-5-1:2023 treats those EMC measures as a system rather than one interchangeable “ground.”
Measure the Installation, Not Just the Transformer
- Draw every conductor. Include both radiator branches, counterpoise or radial conductors, coax exterior, mast, guy wires, bonds, control cables and station wiring.
- Declare the intended return. Mark which conductor should carry antenna current and where that current should end.
- Record R + jX at one calibrated plane. Save the complete load on every intended band rather than only the SWR minimum.
- Map exterior current. Use repeatable probe orientation and marked positions before the candidate choke, after it and along the route to the station.
- Characterise the choke. Record fixture and calibration, complex common-mode impedance, differential insertion/return loss and the exact cable, winding, ferrite and connector construction.
- Change one boundary. Insert or move one choke without simultaneously changing wire, cable route, tuner or bonding.
- Repeat and restore. Use A/B/A trials to expose drift, movement, heating and propagation changes.
- Check the wanted result. Compare accepted power, current distribution, receiver SNR or calibrated field data—not noise floor or SWR alone.
- Verify stress. Measure temperature after equilibrium and recheck the cold electrical response after a representative powered cycle.
Lawrence Livermore National Laboratory’s Numerical Electromagnetics Code can model wires, ground, networks, transmission lines, currents and patterns. Include the coax-exterior branch as a conductor when it participates. A model that stops at the transformer cannot predict a pattern created by the missing return structure.
Primary Technical Sources
- Roy Lewallen, W7EL — Baluns: What They Do and How They Do It: foundational current analysis and experiments on feedline imbalance, coax-exterior current and balun action.
- Larry Lamano, WA0QZY — Common-Mode Current and Common-Mode Chokes: multi-position current measurement and frequency-dependent choke behaviour.
- Tom Rauch, W8JI — End-Fed Half-Wave Matching-System Analysis: modelled and measured end-fed return paths, matching-network loss and the limits of SWR-touch tests.
- Fair-Rite Products — 17th-edition technical catalogue: manufacturer data for complex, frequency-dependent ferrite impedance and material behaviour.
- Lawrence Livermore National Laboratory — Numerical Electromagnetics Code: complete wire, ground, network, current and radiation-pattern modelling.
- IEC TR 61000-5-1:2023: coordinated EMC installation measures including earthing, bonding, cables, shielding, filtering, isolation and surge protection.
Practical Conclusion
I do not rank EFHW and off-centre-fed antennas with a generic CMC table. I draw the actual return conductor, measure the differential load, scan the coax exterior and decide where the antenna should end.
A well-defined EFHW can be a clean installation. A conventional OCF dipole can still excite its feedline. A one-end EF-OCF can deliberately use a section of coax exterior without turning the entire line into an accidental radiator. The difference is not marketing language; it is whether the current path is intentional, bounded and verified.
Mini-FAQ
- Does an EFHW always create more coax-exterior current than an OCFD? No. The result depends on the installed return path, geometry, feedline, matching network, choke and surroundings. Antenna names do not supply a current measurement.
- Can an OCF dipole still have common-mode current? Yes. Feedpoint asymmetry, unequal coupling, the feedline route, supports and nearby conductors can excite an exterior path even when two intended wire arms are present.
- Should an EFHW choke be installed directly at the transformer? Only if the intended return is complete there. If a declared coax-exterior section is the return branch, the choke belongs at that branch’s measured boundary.
- Is 30 dB enough choking for every HF installation? No. Decibels require a defined fixture and measurement. Installed current depends on complex choke impedance plus the exterior loop’s source and load impedances.
- Does low SWR prove that common-mode current is controlled? No. SWR at one reference plane does not reveal exterior current, matching and feedline loss, radiation efficiency or the installed pattern.
- How should I compare two antenna installations? Hold geometry and accepted power constant, map current at several marked positions on every band, change one boundary, restore it, and compare current, stress, SNR or calibrated field evidence.